Electrochromic display devices operate by controlling the redox state of one or more chromophores contained within the device that change color depending on the redox state. These displays are bi-stable over many minutes or hours, only requiring power to update the screen and not to retain information. Electrochromic displays may be direct drive, passive matrix or active matrix devices.
In normal operation of any of these display classes, a voltage or current signal is applied to the device. This causes a change in the charge and hence the coloration of an addressed segment or pixel. In the example described in U.S. Pat. Nos. 6,301,038 and 6,870,657, both of which are incorporated by reference herein in their entirety as if fully set forth, a surface bound redox chromophore is reduced upon application of a negative potential. These devices contain an anode and cathode. In order to balance the charge state on one switching electrode, an electrolyte is provided that contains counter ions of opposite charge to the colored redox state of the chromophore in question. Normally, the counter ion will be free to move within the electrolyte in response to an applied potential. The anion, which provides charge compensation at the anode, and the cation which provides charge compensation at the cathode, move under the action of an emf between the two electrodes. Across two or more differentially charged segments or pixels, however, a potential difference may exist. The potential difference between segments or pixels may cause lateral movement of the charge compensating ions, and thus unwanted crosstalk between segments. Further, the slow equilibration of the counter electrode potential over time due to lateral ionic movement can cause unwanted transient coloration effects.
Crosstalk is commonly considered a risk in the case of passive or active matrix devices, where the pixels are addressed in a multiplexed manner and the pixels may be tightly spaced. Crosstalk is also a risk where the segments are tightly spaced in a fixed segment (direct drive) device. In addition, crosstalk may impact the bistable lifetime in any of the display classes. It is desirable for low-power operation and for enhanced image quality and grey-scale resolution to reduce this crosstalk effect.